Petroleum pipelines are the backbone of this nation's gasoline distribution system, delivering refined fuel products from ports and refineries to storage facilities around the country. Through an intricate network of coordinated deliveries from multiple sources, batches of varying grades and types of fuel travel through these pipelines to predetermined locations. Tank farms are used to store fuel delivered through these pipelines, and to distribute the fuel to other tank farms and ultimately to tanker trucks that deliver the fuel to end use outlets such as retail gasoline stations. Tank farms that simply store the gasoline for further distribution to other tank farms are referred to herein as “intermediate tank farms.” Tank farms that dispense the gasoline to tanker trucks for delivery to the end user are referred to herein as “terminal tank farms.” Petroleum products are typically dispensed to tanker trucks in what is known as a rack, which usually comprise several outlet ports to which different gasoline transports may be coupled. Some tank farms are used for terminal and intermediate functions, and those farms are referred to herein as “combined use tank farms.”
A significant physical property of gasoline is its volatility, or its ability to combust. In order to keep cars performing at a consistent level year-round, gasoline marketers blend agents such as butane with gasoline to increase the Reid vapor pressure and volatility of the gasoline, especially during the colder months. These marketers also add butane to reduce the cost of the gasoline. This blending can occur in-line, so that butane is added directly to a line that is transporting the gasoline. Alternatively, blending can occur in batches, as when butane is added to a storage tank.
Because gasoline vapor emissions from automobiles are a major component of volatile organic compounds (VOC's) in the atmosphere, the United States Environmental Protection Agency (EPA) has promulgated regulations that govern the volatility of gasoline and how much butane can be blended with gasoline. These regulations generally apply from May 1 through September 15, when the gasoline is warmest and most volatile, and require that any blended gasoline be certified for compliance with the volatility guidelines. See 40 C.F.R. §80.27 (2005). The regulations also establish maximum volatility levels for gasoline based on the season of the year, and the region in which the gasoline will be dispensed and sold.
There are two principal methods for assessing the volatility of gasoline: (1) measuring the vapor to liquid ratio, and (2) measuring the vapor pressure. The Reid method is the standard test for measuring the vapor pressure of petroleum products. Reid vapor pressure (RVP) is related to true vapor pressure, but is a more accurate assessment for petroleum products because it considers sample vaporization as well as the presence of water vapor and air in the measuring chamber.
The EPA is also concerned with the sulfur contained in butane, and its emission into the atmosphere when blended gasoline is combusted, and has promulgated regulations that specify how much sulfur is allowed in butane that is added to gasoline, and the testing requirements for assuring that the amounts of sulfur do not exceed the specified amounts. See 40 C.F.R. §§80.195, 80.340(a)(1) (2005). The sulfur content of the butane cannot exceed 30 ppm, and the butane must be sampled at least once every 500,000 gallons of butane to assure compliance. See 40 C.F.R. §80.340(a)(1)-(2) (2005).
Butane is often blended with other gasoline components at the refinery, where it is typically added at the trunk line in response to changes in vapor pressure demand. An exemplary refinery blending process is disclosed in Mayer, U.S. Pat. No. 3,751,644. This patent, which is owned by Sun Oil Company, describes a system for automatically adjusting the amount of butane added to a gasoline stream at a petroleum refinery, based on continuous measurements of the Reid vapor pressure of the gasoline downstream from the point of blending. The described process calculates the amount of butane to be blended based on measurements taken downstream of the blending operation, and does not include measuring the Reid vapor pressure upstream of the blending operation, or calculating the blend ratio based on the Reid vapor pressure upstream from the blending operation.
Bajek's U.S. Pat. No. 3,999,959, which is owned by Universal Oil Products Company, also discloses a system for blending butane and gasoline at a petroleum refinery. The Bajek system blends butane with a low-octane gasoline stream and a high-octane gasoline stream, and then analyzes the blended gasoline to measure characteristics such as Reid vapor pressure and vapor to liquid ratio. Bajek does not disclose monitoring the gasoline upstream of the blending operation, or calculating the blend ratio based upon such upstream monitoring.
Efforts at blending butane at a terminal tank farm have also recently been undertaken. As described in our granted patent U.S. Pat. No. 6,679,302 (to which this application claims priority), butane can be blended in-line with a gasoline stream immediately before the gasoline is dispensed to a tanker truck, and after it has been withdrawn from the storage tank. In a preferred process described in this patent, the Reid vapor pressure is measured upstream of the blending operation, and the blend ratio is calculated based on the upstream measurement.
Lastly, the inventors are aware of an unpatented system that is used to blend butane and gasoline at several terminal tank farms. These systems continuously monitor the Reid vapor pressure of gasoline that is introduced to a storage tank, and blend butane with the gasoline based upon the vapor pressure measurements. These systems do not continuously monitor the Reid vapor pressure downstream of the blending operation as an integrity check. Instead, they certify the integrity of the blending operation by periodically measuring the Reid vapor pressure of the entire storage tank.
Several methods have been attempted to improve the precision of butane blending and the predictability of Reid vapor pressure in the final product. The Grabner unit is a substantial advance in this respect. The Grabner unit (manufactured by Grabner Instruments) is a measuring device capable of providing Reid vapor pressure and vapor to liquid ratio data for a gasoline sample typically within 6-11 minutes of introducing the sample to the unit. It has been employed at some refineries to consistently measure the volatility of gasoline, and to blend butane with the gasoline based upon an allowable Reid vapor pressure for the gasoline.